Molecular beam epitaxy is an important aspect of the manufacture of semiconductor electronic devices such as integrated circuits and transistors for use at U.H.F. and microwave frequencies. The composition, thickness, and uniformity of the deposited layers largely determines the performance of devices manufactured in this way, and it is consequently important to be able to control these parameters accurately during the deposition process, both for producticn and research purposes. Several techniques for depositing thin films are known, but that known as Molecular Beam Epitaxy is one of the most precise and versatile. It involves the growth of epitaxial (single crystal) films from directed thermal energy molecular or atomic beams on a crystalline substrate in ultra high vacuum (less than 10.sup.-9 torr). An important feature of the process is clearly the production of atomic or mclecular beams of the substances to be deposited, and this invention is concerned with the type of source known as a Knusden Cell. In their simplest form these consist of a crucible of 10-40 ccs. capacity enclosed in a small oven, into which a sample of the required material is placed so that it can be inserted into the vacuum system and vaporized in a controlled way to produce the atomic or molecular beam. The mouth of the crucible is often restricted by an orifice which has a diameter less than the mean free path of the molecules at the pressure in the vacuum system. This results in the production of an atomic or molecular beam in which the direction of motion of the molecules is collimated in one direction, and in which the number of intermolecular or interatomic collisions is minimized. Knusden cell sources are conventionally used for the production of atomic or molecular beams of arsenic, gallium, phosphorus, antimony, indium, manganese, germanium, tin, zinc sulphide, lead telluride, and other similar materials.
A conventional Knusden cell consists of a crucible into which a solid sample of the material is introduced, and a heater capable of raising the temperature of the crucible to between 350.degree. C. and 1400.degree. C., dependent on the material to be vaporised. A remotely controlled shutter of a refractory metal such as tantalum is often fitted over the mouth of the crucible to prevent the escape of the molecular beam when it is not required. The entire cell is mounted in an ultra high vacuum apparatus, and has to be constructed with this in mind. The materials used must be such that even at 1400.degree. C. they do not produce significant amounts of vapour either from the decomposition of the cell materials themselves, or from the expulsion of other materials such as water absorbed into the cell before it was fitted into the vacuum system. Any such contamination emitted from the cell when it is heated will be detrimental to the purity of the thin film deposited from the cell, as well as reducing the vacuum that can be attained in the apparatus. Great care is therefore necessary in the design and construction of these cells to minimize the amount of contamination which reaches the substrate on which the layer is being deposited. Generally, the crucible is made from pyrolytic boron nitride (typically 99.999% pure) or from spectroscopically pure graphite, and the heating elements from pure tantalum wire which is helically wound round the crucible. Several heat shields made from tantalum foil may be wound over the heater, and the entire cell is often surrounded by a cooling shroud which prevents heat escaping from the cell and heating other parts of the vacuum system, as well as condensing any contaminating material vaporised from the crucible walls or the heater. A restricting orifice may also be fitted to the mouth of the crucible as described above. Knusden cells constructed in this way are known, and are briefly described in UK patent application No. 2,012,818 A and UK patent No. 1,469,978.
An alternative form of construction is to employ a heater consisting of tantalum wires running parallel to the axis of the crucible. These wires are conventionally enclosed in alumina tubes to support them when they are heated. Cells constructed in this way are more suited to use at lower temperatures, for the reasons described below. Both these forms of construction suffer from important disadvantages because the area of contact between the wire and the crucible is low, and the heat transfer from the wire to the crucible is inefficient. This requires the temperature of the heating wire to be considerably higher than the required temperature of the crucible, which increases the contamination emitted from the heater and its supports, and limits the maximum temperature to which the crucible can be heated without premature failure of the heater. The large proportion of waste heat radiated from a cell of this kind also increases the demands on the cooling system for the cell shroud. If the heating wire is spaced away from the cell wall, as for example in the alternative form of construction, the efficiency of the heat transfer falls still further, and the problems of supporting the heating element are increased. In the example given above, alumina tubes are employed. However, the heating wire will be in contact with these tubes, and because its temperature has to be considerably higher than that of the crucible, the contamination emitted from the cell will be correspondingly increased. For this reason, cells constructed in this way are more suited to use at lower temperatures. The use of pyrolytic boron nitride tubes to enclose the heating elements would reduce the amount of contamination, but it is a very expensive material, and the cost of the cell would be greatly increased. A further disadvantage is that a heater formed in this way usually contains a number of sharp bends in the wire, which are likely to result in its premature failure. Replacement of the wire is also difficult. The use of a helically wound element on the crucible does not eliminate the requirement for support insulators in contact with the wire because the expansion of the wire when it is heated would result in it becoming slack on the crucible and moving out of position unless additional supports are provided. These increase the amount of contamination, as discussed above. The use of a helically wound element also introduces another disadvantage, namely the existence of a cyclic temperature gradient along the axis of the crucible with the points of maximum temperature at the points where the wire element is in contact with the crucible wall. This is emphasised by the relatively poor thermal conductivity of pyrolytic boron nitride.
However, the most serious defect affecting both types of cell is the fall in temperature at the mouth of the crucible due to the greater heat loss by radiation from the end of the crucible which cannot be enclosed by the heating element. This problem is increased when the mouth is restricted to produce true Knusden evaporation. The effective evaporation temperature of the cell is clearly the lowest temperature in it, because material evaporated at higher temperatures will condense at this point. The fall in temperature at the mouth means that the bulk of the crucible must be heated to a still higher temperature than that needed to evaporate the contents to avoid condensation in the mouth, and to maintain the desired flux of atoms or molecules. This defect further limits the effective maximum temperature of the cell. In some cases it becomes so severe that it is necessary to remove the restriction in the mouth of the cell in order to achieve adequate deposition rates. A wide-mouthed cell of this type does not behave as a true Knusden cell, and the atomic or molecular beams produced are not as well collimated, resulting in a greater spread of the beam and a consequent reduction in the deposition rate on the substrate and a corresponding increase in the contamination of the rest of the vacuum system. However, up to now it has been necessary to tolerate these defects in order to produce a molecular beam at the desired temperature, and many sources for molecular beam epitaxy are designed without the narrow orifice because of the difficulty of heating the end plate which would contain it. Some improvement can be made in the case of a helically wound heater by decreasing the pitch of the turns at the mouth of the crucible, which increases the heat input at this point and reduces the fall in temperature. However, this necessarily means that the pitch of the turns round the remainder of the crucible is greater, and this again limits the maximum temperature as well as worsening the temperature gradient between the turns inherent with a helical heater. In practice, a combination of these problems limits the maximum working temperature of conventional cells to about 1300.degree. to 1400.degree. C., which means that they cannot be used to produce beams of silicon or iron, and are not particularly satisfactory with germanium. If a source operating with true Knusden evaporation is used. its maximum effective temperature may well be lower than 1300.degree. C.
It is an object of the present invention, therefore, to provide a Knusden cell for molecular beam epitaxy in which these difficulties are overcome, which results in deposited layers of greater purity and uniformity, and which can be used at higher temperatures than conventional cells so that, for example, beams of silicon and iron can be produced without the need for expensive processes like electron beam heating. It is a further object to provide a cell in which the crucible can easily be changed in case of damage, or substituted for one of a different volume, without substantial changes to the rest of the cell, and one in which the heating elements are robust and easily changed in case of failure. It is also an object of the present invention to provide a heating element for a Knusden cell which can produce a higher heat input to a particular part or parts of the crucible than to the remainder, so that the temperature gradient along the cell can be adjusted, usually to minimise it, therefore allowing the construction of a cell capable of true Knusden evaporation at higher temperatures than previously possible.